This invention relates to a circuit arrangement for evaluating the electrical output signals of a detector for thickness changes in a fuel injection line under the influence of an injection process in general, and more particularly to such a circuit for evaluating the rapid pressure variations generated when an injection valve is closed.
An injection time detector for internal combustion engines having fuel injection which comprises a transducer attached to the outside of the fuel injection line for picking up the elastic changes of the outside diameter of the line caused by the injection process and converting them into an electrical signal is disclosed in copending application Ser. No. 715,850. The injection process lasts a certain time. It is therefore important to select from the electrical signal representing the entire injection process a part which always appears, within the overall process, at the same point in the injection process, regardless of the duration of the injection. One possibility for this purpose is the beginning of the injection process. The relatively flat slope of the injection pressure at the beginning of the injection process is, however, disadvantageous and makes such a choice questionable. A further problem in measuring is that, even if the detector is attached only to the injection line of one cylinder, signals above the general noise level from the injection lines of the other cylinders are also transmitted to the detector, even if attenuated.
Investigations of the entire injection process have shown that a pressure waveform similar to a water shock ("hammer") caused by the rapid closing of the injection valves, occurs in the fuel. This waveform is also transmitted to the walls of the fuel injection line. The waveform is a sequence of relatively high frequency (&gt;500 Hz) pressure waves with, first, a high and then rapidly decreasing amplitude.